1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and more specifically to high frequency, switched mode power electronic converter circuits.
2. Description of Related Art
There are some power conversion circuits which accomplish power conversion from one DC voltage level to another DC voltage level using an intermediate energy storage mechanism such as a power inductor. Examples of such power conversion circuits include the buck, boost, and buck boost converters which are well known to those skilled in the art of power conversion. These converters are simple, requiring only one large magnetic storage element, the power inductor, and two switches. Typically input and output capacitors are added to the basic circuit. For isolated power conversion an isolation transformer or a coupled inductor is required in addition to or instead of the power inductor. One problem associated with practical non-isolated power conversion circuits that employ the buck, boost, or buck boost converters is that either the input current, the output current, or both the input and output current are pulsating, that is non-continuous. The pulsating currents result in high ripple currents and high ripple voltages by comparison to inputs or outputs in which the input or output current is continuous or non-pulsating. In order to reduce the effects of the pulsating currents an additional filter stage using an additional inductor and capacitor, or LC combination, or the use of a much larger input or output capacitor is required. The consequence of the additional filtering is higher cost and, in the case of the LC filter, higher losses. For example, the buck converter has a pulsating (discontinuous) input current and a non-pulsating (continuous) output current. Power supplies that use buck converters often employ an LC input filter to reduce the ripple resulting from the pulsating input current. One specific example is the Datel dual output BMP models which use a buck post regulator with an LC input filter, which is illustrated in FIG. 1, to generate a second power supply output. The BMP models employ the main output of the power supply as the input to the buck post regulator. If they did not use an input LC filter with the buck post regulator the pulsating input current of the buck post regulator would compromise the ripple performance of the converter's main output. One example of prior art that overcomes the problem is the famous Cuk converter, shown in FIG. 2, which is also referred to by its inventor as the optimal topology converter. The Cuk converter circuit is optimal in the sense that both the input current and output current are non-pulsating. One of the likely reasons that the Cuk converter was not chosen in the Datel design is that the non-isolated Cuk converter has an output voltage that is opposite in sign to its input. Another shortcoming of the Cuk converter is that its control to output transfer function has a right half plane zero which, in practical terms, limits the available bandwidth, which in some cases, especially where fast load current transient response is a requirement, renders the Cuk converter unsuitable. Cuk, in a paper included in his book entitled "Advances in Switched-Mode Power Conversion" on page 319 explains how the re-orientation or rotation of the switching cell in the Cuk converter leads to buck and boost converters with continuous input and output currents. The continuous input and output current buck and boost converters that Cuk describes in his book on page 319 are simply the well known buck converter with an LC input filter, shown in FIG. 1, and the well known boost converter with an LC output filter, respectively. The Datel design is then the buck implementation of the Cuk converter as described by Cuk in his own writings. The same process of generating continuous input and output current buck and boost converters from the Cuk switching cell is also described in a recent book by Robert Erickson entitled "Fundamentals of Power Electronics" on pages 141 and 142. In Erickson's book he describes a process for generating the buck and boost forms of the Cuk converter similar to the process described by Cuk with identical results. There is one important feature of the Cuk converter which is missing in the buck and boost derivations described by Cuk and Erickson. The Cuk described buck and boost derivations both require two separate inductors which cannot be magnetically coupled. This requirement is due to the fact that the AC wave forms of the two inductors are dissimilar, whereas the AC wave forms of the two inductors in the Cuk converter are very similar or identical which enables the combination of the two chokes of the Cuk converter in a single simple magnetically coupled inductor construction. In the non-pulsating buck and boost converters described by Cuk and Erickson one of the chokes has a large AC winding voltage component and the other has a very small AC winding voltage component. In the same book by Cuk on page 337 Cuk describes how his optimal topology converter works when implemented with the two chokes combined in a single simple coupled inductor, as illustrated in FIG. 3. Several patents have been issued on the basic Cuk converter and the coupled inductor versions of the Cuk converter. What is needed are simple optimal topology converters in which the two inductors can be combined into a single simple magnetically coupled inductor and can accomplish step up or step down DC to DC conversion without inversion or isolating transformer. Such an optimal topology converter in the buck form would reduce the size and cost of converters of the Datel BMP type by combining the two inductors into a simple single coupled magnetic circuit element. Such an optimal topology converter in the boost form would satisfy a need for converters similar to the Datel BMP type, but where the main output voltage is lower than the voltage of the second output.